a) Field of the Invention
The present invention relates to a tin-silver-based soldering alloy and, more particularly, to a soldering alloy having a fusing property equivalent to or higher than that of "Alloy H" and which has excellent mechanical properties such as tensile strength and elongation value.
b) Description of the Prior Art
Conventionally, an alloy having an eutectic composition near Pb-Sn has been known as a typical soldering alloy. Further, an alloy made of Zn--Cd having a higher strength than that of Pb--Sn eutectic soldering alloy has also been known. However, the former soldering alloy has a problem of toxicity of lead and the latter soldering alloy has a problem of an adverse effect of cadmium evaporation to an operator. Therefore, neither has solved these recent environmental problems.
Under the above circumstances, various types of silver-tin-based soldering alloys containing no harmful Pb or Cd have been proposed. For example, "Alloy H" (manufactured by Nippon Alpha Metals Co., Ltd.) comprising Ag 2.0%, Cu 0.5%, Bi 7.5%, and Sn as the remainder, is known. Alloy H has a melting point which is higher than that of the Pb--Sn eutectic solder and lower than that of the conventional silver-tin-based soldering alloy containing no Pb or Cd, i.e., 212.degree. C., and thus has excellent fusing properties. However, Alloy H has a low elongation value in a tensile test. Accordingly, there has been the risk that, when a heat cycle is applied to a soldered portion, it is not possible to absorb the thermal expansion difference between a substrate and a part so that the soldered portion might be broken.
The inventors of the present invention made various investigations on a soldering alloy which does not include harmful Pb or Cd and which is excellent in tensile strength and elongation value. As a result, the inventors made the invention disclosed in Japanese Patent Application Laid-Open No. 8-187590. The invention described in Japanese Patent Application Laid-Open No. 8-187590 provides a soldering alloy which has a tensile strength and an elongation value almost equivalent to those of the eutectic soldering alloys as well as an excellent melting point property. However, the inventors have further found another soldering alloy having excellent properties beyond the scope of the invention of Japanese Patent Application Laid-Open No. 8-187590, and thus the present invention has been made. In general, when the melting point property deteriorates, the soldering temperature must be set high, which has risks of thermally damaging the part to be soldered and shortening the product life.